Summary A de Havilland DHC-6-100 float-equipped Twin Otter (serial number 086), operated by West Coast Air Ltd., was on a regularly scheduled flight as Coast 608 from Vancouver Harbour water aerodrome, British Columbia, to Victoria Harbour water aerodrome. The flight departed at about 1510 Pacific standard time with two crew members and 15passengers on board. Shortly after lift-off, there was a loud bang and a noise similar to gravel hitting the aircraft. Simultaneously, a flame emanated from the forward section of the No2 (right-hand) engine, and this engine completely lost propulsion. The aircraft altitude was estimated to be between 50 and 100feet at the time. The aircraft struck the water about 25seconds later in a nose-down, right wing-low attitude. The right-hand float and wing both detached from the fuselage at impact. The aircraft remained upright and partially submerged while the occupants evacuated the cabin through the main entrance door and the two pilot doors. They then congregated on top of the left wing and fuselage. Within minutes, several vessels, including a public transit SeaBus, arrived at the scene. The SeaBus deployed an inflatable raft for the occupants, and they were taken ashore by several vessels and transported to hospital for observation. There were no serious injuries. The aircraft subsequently sank. All of the wreckage was recovered within five days. Ce rapport est galement disponible en franais. Other Factual Information The accident flight was the fifth take-off of the day for the crew in this aircraft. Before the engines were started, passengers received a safety briefing. Normal pre-flight checks and pilot briefings were completed in accordance with the operator's standard operating procedures (SOPs)1. Take-off proceeded normally as the aircraft lifted off and began to climb. As the aircraft approached 70knots indicated airspeed (KIAS), loss of propulsion occurred causing a sudden yaw to the right. The pilot flying immediately applied full left rudder. Both crew members acknowledged a loss of engine power and agreed to land straight ahead. The pilots selected full flaps, confirmed that the No2engine (Pratt Whitney PT6A-20, serial number 21867) was the affected engine, pulled the fire extinguisher T-handle, and activated the fuel emergency shut-off valve. Take-off power was maintained on the No1engine until power was intentionally reduced immediately before impact. The aircraft was not equipped with an optional auto-feather system, and the crew did not attempt to manually feather the right-hand propeller. At impact, the right wing separated from the fuselage at the wing root, but cables or other miscellaneous materials kept the two sections together as the wreckage sank. The right float also separated at impact and was recovered before it sank. All of the recovered wreckage was found at a single site. The aircraft was departing under visual meteorological conditions on a northeasterly heading with an easterly wind of 5 to 10knots. General area weather recorded at 1500 Pacific standard time2 for the Vancouver International Airport, seven nautical miles south of Vancouver Harbour, was as follows: wind 150true (130magnetic) at 4knots, a few clouds at 4000feet, and higher, broken cloud layers. Weather did not contribute to this occurrence. The calculated take-off weight and centre of gravity of the aircraft, 10736pounds and 28% MAC respectively, were within the take-off limits of 11600pounds and 25 to 32% MAC. The aircraft was equipped with conventional floats and configured for a commuter operation with 18passenger seats. The aircraft's Universal 30A cockpit voice recorder was recovered and provided meaningful information. No flight data recorder was installed, nor was it required by regulation. The aircraft was equipped with an engine-fire detection and extinguisher system. The engine fire warning system did not activate in response to the internal engine fire, nor should it have by design. The flight crew discharged the engine fire extinguisher when they saw the reflection of flames emanating from the area of the No2engine. The crew would not have known that the fire extinguisher would not be effective. The internal engine fire would likely have been short-lived once the engine stopped running. The fire was extinguished completely by water immersion at impact. The balance of the aircraft was unaffected by fire. Post-accident inspection of the No2 engine revealed that the power turbine had disconnected from the propeller because a first-stage planetary gear in the propeller reduction gear box (RGB) had fractured and disintegrated, causing extensive internal damage. Once the disconnect occurred, the power turbine disk oversped and shed all of its blades, as designed. Metallurgical analysis showed that fracturing of the planetary gear resulted from damage on the surface of the gear bore. This damage was caused by the bearing sleeve rotating within the gear bore. The most probable scenario is that the bearing sleeve bound to the associated planetary gear shaft (axle) because small pieces of wire from the individual planetary gear oil strainer screen contaminated the lubrication oil and entered the bearing sleeve / planetary gear shaft (axle) interface. The binding of these two surfaces caused the bearing sleeve to rotate within the gear bore. The engine and RGB were manufactured by Pratt Whitney Canada Corp. (PWC). Representatives of the manufacturer participated in a TSB supervised tear-down and analysis. Pratt Whitney report number TL-1626 concluded the following: The planet gear oil strainer screen exhibited a deformed Teflon insert, pinched Oring, bent neck and fretting of the strainer housing suggesting an improper installation of the strainer within the pin. The screen beneath the outer screen exhibited fractures in the braze and wires. The wires fractured by fatigue. Some wire fracture surfaces were smeared and corroded suggesting the fractures were not recent, and as a result, the fracture of the strainer is believed to be a consequence of the installation. The release of wire fragments and debris may have subsequently initiated or contributed to the distress of the bearing sleeve. Each engine on the aircraft was equipped with a magnetic engine oil chip detector installed in the RGB casing. The detector will attract any magnetic metal particles in the engine oil (the same oil lubricates the RGB). A collection of metal particles will make a contact between two terminals in the detector, thereby establishing continuity, which can be tested by maintenance personnel. This aircraft was not manufactured with flight instrument panel-mounted annunciators connected to the chip detectors, nor was it required by regulations. The annunciator system is available as a retro-fit package; however, without this package, detection is limited to visual inspection. Each engine on the aircraft was also equipped with a fully feathering, reversible pitch propeller. An optional auto-feather system was not installed, nor was it required by regulation. An auto-feather system is armed when prescribed criteria are met; if the system detects a difference in torque between the two propulsion systems, it automatically feathers the propeller blades on the side with reduced power, thereby reducing the aerodynamic drag of the windmilling propeller. This function can significantly reduce the pilot workload required to maintain control of an aircraft in an asymetric thrust condition. West Coast Air maintenance records indicated that the No2 engine was last overhauled by PWC Aircraft Services Inc. of Bridgeport, West Virginia in 1993. During overhaul by an approved facility is the only time a RGB is disassembled. At that time, the planetary gear oil strainers were replaced. (See Appendix A.) Since then, the engine had operated about 3235 hours. Regular RGB maintenance consists of prescribed inspections and was performed by West Coast Air as part of the de Havilland Equalized Maintenance Maximum Availability (EMMA) program. The EMMA program required a continuity check of each magnetic chip detector at 100-hour intervals and removal of the chip detectors for visual inspection at 600 hour intervals. West Coast Air records showed that a regularly scheduled 100-hour inspection had been completed about 46 hours before the accident flight. Information provided by West Coast Air indicates that their 100-hour inspections exceed requirements by including removal of each chip detector for visual as well as continuity checks and submission of an oil sample to a separate agency for analysis. Analysis of the last oil sample did not indicate any change in wear-metal readings from previous samples. Post-accident metallurgical analysis resulted in the assessment that wire fractures in the affected planet gear oil strainer were not recent, and therefore, this unsafe condition had developed over a longer period. However, there was no detectable evidence of a pending problem at the last inspection and the result of this unsafe condition occurred in less than 46 hours of operating time. The DHC-6 becomes airborne at 55 to65 KIAS, varying with the aircraft weight. The velocity of minimum control (Vmc) for single-engine operation is 66KIAS with 20 of flaps set, which is the only approved flap setting for take-off on floats. The Transport Canada-approved aircraft flight manual (AFM) recommends lifting off at about 60KIAS and accelerating to a target speed of 76KIAS at 50feet above ground level when taking off at gross take-off weight. The West Coast Air SOPs indicate that normal take-off procedure is to lift off at 55 to 65 KIAS and accelerate to 70KIAS, at which time a speed call is made by the pilot not flying. At a confirmed positive rate of climb, flaps will be retracted from 20 to 10. During flap retraction, acceleration will continue to a target speed of 76KIAS at 50feet. At 76KIAS, the focus changes from acceleration to altitude gain. Another call is made at 400feet, at which time flaps are fully retracted and speed is increased to 83KIAS. The first power reduction and after take-off checks commence at this time. The pilot's SOP emergency procedures briefing prescribed that if a safety-related problem occurred before lift-off, or after lift-off with sufficient water (distance) remaining, the pilot flying would reject the take-off. Neither the AFM (which is incorporated into the operator's SOPs), nor other sections of the operator's SOPs describe a procedure for rejecting a take-off in a float operation when indicated airspeed is above Vmc. The AFM-recommended emergency procedure for rejecting a take-off due to a power loss while airborne and at a speed below Vmc is to reduce power on the operating engine sufficiently to assure control of the airplane and land straight ahead.3 The AFM emergency operating procedures section prescribes the following actions if a power loss occurs when airborne but at a speed below Vmc:4 Power levers - IDLE Land straight ahead . . . Note: If time permits, fuel levers OFF, DC master switch OFF. The Canadian Aviation Regulations (CARs) do not require pilot proficiency training to be completed in a flight simulator for non-pressurized, turboprop-powered aircraft. The only full flight simulator for the DHC-6 is located in Toronto and is configured for a wheel equipped aircraft; neither pilot had received DHC-6 emergency training in a simulator. The operator's Transport Canada-approved pilot training syllabus incorporates training in the aircraft and includes emergency scenarios and procedures during and after take-off. However, the scope of exercises that can be carried out in the aircraft is inherently limited because of safety concerns. The pilot-in-command held a valid Canadian airline transport pilot licence - aeroplane (ATPL-A) and had a current pilot proficiency check for the DHC-6. He had logged about 4000 hours' total flight time, with about 2500hours logged on the DHC-6. He had been employed by the operator since April 2000. The first officer held a valid Canadian commercial pilot licence - aeroplane (CPL-A) and had a current pilot proficiency check for the DHC-6. He had logged about 2000 hours' total flight time, with about 1650 hours logged on the DHC-6. He had been employed by the operator for about three years and as a pilot since January 1999. Each pilot was wearing a seat lap belt with two shoulder straps. Neither pilot was injured, and both were able to help passengers evacuate. All passengers were able to exit the aircraft through the main entrance door and the two pilot doors. Although no serious injuries occurred, the two passengers seated in the first row hit their heads on the forward bulkhead behind the pilot seats during impact. About half of the passengers donned their life vests. Several passengers found the life vests hard to reach under the seat cushion. Each seat on board was equipped with a life vest; regulations require a life vest for each person on board.